We encountered a case of suspected delayed-type allergy to lidocaine. The patient was a 60-year-old woman who had previously undergone dental treatments with methylparaben-free lidocaine at least twice without experiencing any allergy symptoms. However, the following year, significant swelling of the vulva was observed in the evening after the morning of the application of an antipruritic cream to that area. Three months later, significant swelling and redness of the lips and chin were also observed at least several hours after the use of a local anesthesia to treat left nasal bleeding. At that time, a lidocaine allergy was diagnosed by a dermatologist. We initially performed a patch test based on our suspicion that methylparaben constituted the antigen component of the delayed-type allergy. Test reagents were applied with a patch tester kit to the anterior region of the forearm, and the kit was removed after 48 h. The evaluation was performed according to the standards of the International Contact Dermatitis Research Group at 49 and 72 h after the application of the tester kit. Test numbers 1, 3, and 4 were all strongly positive (+ +), indicating the presence of erythema, infiltration, and papules containing vesicles, regardless of the inclusion of methylparaben. As a result, an allergy to lidocaine was strongly suspected, contrary to our expectations. We next performed a patch test using procaine and obtained negative reactions. The patient subsequently underwent further dental treatments using procaine without experiencing any allergic reactions. These findings indicate that a drug may become an allergen, even if it has low antigenicity and has been used safely in the past.